Time Division Multiple Access (TDMA) radio communication systems provide integrated communication, navigation, identification and position location means for military operations.
Tactical radio communications in an electronic warfare (EW) environment must be made as immune as possible from all EW measures such as high power brute force jammers, spoof jammers, electronic signal interceptors, and direction finders. Simply increasing friendly transmitter power is expensive as well as ineffective against spoof jammers, interceptors, and direction finders. In Time Division Multiple Access (TDMA) the systems such as JTIDS, the functions of communications relaying, navigation/position location, and identification require that each user maintain alertness to all other user transmissions. This requirement rules out the use of highly directional antennas to provide anti-jam protection. Consequently, jam resistant, intercept-resistant phase modulated, phase noncoherent spread spectrum Direct Sequence Pseudonoise (DSPN) communication links such as Joint Tactical Information Distribution System (JTIDS) have been developed for TDMA communications.
JTIDS is a notable example of a communications system which utilizes a phase modulated, phase noncoherent radio signal with antijam capabilities. JTIDS is an integrated communications, navigation, and identification TDMA military communications system designed to operate in a 960-1215 MHz TACAN band and in a CW tactical environment spanning a geographic area of up to 500 miles. The time division JTIDS architecture allows multiple users to participate in the communications network. The various terminals of the JTIDS network may comprise, for example, mission aircraft subscribers, sea based subscribers and ground subscribers. All terminals in the network can listen to all time divisions or time slots in which they are not transmitting. Code division mulitiplexing is used to provide multiple communication networks which have overlapping coverage patterns. A terminal can switch between multiple networks on a time slot by time slot basis. Each 7.8125 millisecond time slot contains a 3354-microsecond transmission burst comprising 129 symbols, each symbol having a duration of 26 microseconds. Symbols consist of either one or two 32-chip pulses, each of a duration of 6.4 microseconds.
In the EW environment, the possibility of multi-tone CW jamming based on frequency modulating techniques poses a threat to TDMA communications not heretofore adequately addressed by current signal processing procedures. The TDMA signal is currently processed by an MSK chip matched filter followed by an IAGC, such as the MITRE IAGC, which may be viewed as a hard limiter followed by a three bit phase quantizer. The phase quantizer is followed by a signal correlator, which in the JTIDS processor comprises a 32 chip pulse correlator. At the signal receiver input, the DSPN information signal plus CW and Gaussian interference signal enter the chip matched filter at a jammer power to signal power ratio of J/S. The chip matched filter serves to maximize the ratio of signal power to the Gaussian component of interference. At the hard limiting step, the MITRE IAGC provides the signal processing with a property called constant false alarm rate (CFAR) wherein any sudden high power burst of spurious jamming or noise entering the system will be clipped to a preset amplitude level and thereby prevented from propagating further into the chain of signal processors. Furthermore, the probability of declaring a noise symbol to be a signal symbol remains constant, independent of the magnitude of that high power burst of jamming. Hard limiting, however, also introduces what is known as a small signal suppression effect which degrades the signal to noise ratio of the signal entering the correlator. Additionally, in the case of the JTIDS signal, the noise equivalent bandwidth of the chip matched filter is a fraction .sup..pi.2 /16, of the chip rate or 0.62 times the chip rate, which is equivalent to -2.1 dB. Consequently, J/S increases by 2.1 dB as the input signal together with a band-centered CW jammer pass through the matched filter. As the matched filtered input signal passes through the hard limiter, the effects of small signal suppression in the hard limiter result in a further increase in J/S of about 4.3 dB (greater than 4.3 dB if J/S at the hard limiter input is greater than 2.1 dB). Since the J/S conversion loss in the order of 4.3 dB represents a 2.7 to 1 loss in relation to transmitter power, it constitutes a substantial deficiency in JTIDS and other phase modulated, phase noncoherent signal processing systems not heretofore adequately addressed in the prior art.